The present invention relates to a power steering system of a vehicle, and more particularly, to a motor driven power steering (MDPS) system and a control method thereof.
A power steering system of a vehicle is a steering device based on power, and serves to assist a driver to operate a steering wheel. Conventionally, a hydraulic power steering system has been used as the power steering. Recently, however, the use of an MDPS system which uses a motor force has increased. That is because the MDPS system has a smaller weight and occupies a smaller area than the existing hydraulic power steering system, and does not require oil change.
The MDPS system provides a part of torque which a driver must apply to a steering wheel during a steering operation, using an auxiliary power source, thereby enabling the driver to easily perform the steering operation. That is, a torque sensor connected to the steering wheel senses a steering intention of the driver, and the MDPS system receives the sensing signal and drives a motor to provide a suitable force in consideration of the current speed of the vehicle, thereby assisting the steering force. In general, during a parking operation or low-speed operation of the vehicle, the MDPS system provides a large force to lighten the force of the driver. During a high-speed operation, however, the MDPS system provides only a small force to maintain the stability of the vehicle body.
The MDPS system includes various types of control logic such as logic for realizing a steering feel desired by a driver, logic for improving the stability of the vehicle, and logic for improving the stability of the system. However, based on the basic principle that a torque value sensed through a torque sensor is inputted to a boost function to calculate a basic assist torque, the control logic of the MDPS system determines the final assist force by adding or subtracting various compensation values to or from the assist torque.
The related art of the present invention is disclosed in Korean Patent Laid-open Publication No. 10-2010-0056312 published on May 27, 2010.
In such an MDPS system, when a load is changed by disturbance, the load change causes a variation in torque caused by a driver. The load indicates a value which maintains the equilibrium with the sum of the torque by the driver and the assist torque. At this time, an assist force to overcome the variation is generated in proportion to a boost gain (the slope of the boost function). Thus, as the boost gain is increased, the MDPS system is resistant to disturbance.
However, in the conventional assist torque calculation method, when the target value of steering feel is set, a boost gain for each torque value is determined. The steering feel indicates how heavily a driver feels steering. Thus, since the boost gain cannot be freely controlled, the MDPS system is vulnerable to disturbance.
Furthermore, since the relation between the target value of the steering feel and the boost gain is not established, the boost gain is not calculated according to the target value of the steering feel, but determined through a tuning process in which changing the boost gain and checking the steering feel based on the changed boost gain are repeated. Thus, the system tuning process requires a large amount of time, and the precision of the system tuning process is reduced.